It is known to use two kinds of electron sources in an electron beam device: the first kind is a thermionic source which emits electrons when heated. The second kind is a field-emission source which emits electrons when an intense electric field is applied to it.
A thermionic source uses, for example, a tungsten filament, a pointed emitter of a single crystal or a sintered compound of lanthanum hexaboride (LaB6) or cerium hexaboride (CeB6). If those materials are heated to a sufficiently high temperature, the electrons of the materials receive sufficient energy to overcome the natural barrier (work function). Therefore, the thermionic source is caused to emit thermally excited electrons, thereby generating an electron beam.
The design of a thermionic source can vary. It is known to use a thermionic source comprising a tip, for example a tip of a tungsten filament or a LaB6 crystal comprising such a tip. Furthermore, it is also known to use a LaB6 crystal in the form of a truncated cone with a polished circular disk. This disk is an electron emission surface.
When using thermionic sources like tungsten and LaB6, a thermionic source is used in a triode gun. This triode gun comprises an electron source (thermionic source) in the form of a cathode, a so-called Wehnelt cylinder and an anode with an aperture in its centre. A high voltage is placed between the electron source and the anode, modified by a potential on the Wehnelt cylinder which regulates the emission current and focuses the electrons into a crossover having a diameter and convergence angle α. Therefore, the crossover is a point at which the electrons emitted by the electron source converge.
A field-emission source operates according to a different principle than thermionic sources. The principle behind field-emission is that the strength of an electric field is considerably increased at sharp points. If the electric field is high enough, the work function barrier is sufficiently lowered for electrons to tunnel out of the material due to the tunnelling effect or the Schottky effect.
There are two types of field-emission sources, namely a cold field-emission source and a thermal field-emission source. In the case of a cold field-emission source, the end of an electron source is normally made from a single crystal fine tungsten wire and is subjected to a strong electric field at room temperature whereby electrons in the single crystal are emitted using a tunnelling effect, so that an electron beam is generated. However, to allow field-emission, the surface must be free of contaminants and oxides. This can be achieved by operating the system under relatively good vacuum conditions (for example, the residual pressure being lower than 10−9 mbar), in particular under UHV conditions (ultra high vacuum conditions). In the case of a thermal field-emission source, the electron source is heated while being subjected to a strong electric field which causes electrons to be emitted using the Schottky effect, so that an electron beam is generated. The required vacuum conditions for such an electron source are more relaxed, but still require a residual pressure lower than 10−8 mbar and, therefore, are still UHV conditions.
Both field-emission sources have to be operated under good vacuum conditions. This is a disadvantage because the time, effort and costs for providing such good conditions are relatively high. Thermal field-emission sources have a further disadvantage due to their relatively larger energy spread with respect to the electrons emitted from the electron source.
With respect to the above mentioned prior art, reference is made to GB 2 389 450 A, EP 1 947 674 A1 as well as WO 2008/001030 A1, all of which are incorporated herein by reference.
In a case where an analysis is carried out in a small region, an electron beam with a high level of brightness is required in order to reduce the diameter thereof. The brightness of an electron beam is the current density (number of electrons per unit area per unit time) per unit solid angle of an electron source. The brightness β of an electron source is calculated by
                              β          =                                    I              Beam                                      π              ·                              α                2                            ·              π              ·                                                r                  eff                                2                                                    ,                            [                  Equation          ⁢                                          ⁢          1                ]            where IBeam is the beam current, α is the beam semi-angle (all electrons contributing to the beam current IBeam are emitted by the electron source within the semi-angle α) and reff is the effective radius of the electron source. The effective radius reff is given byreff=√{square root over (r02+δrS2+δrC2)}  [Equation 2],where r0 is the aberration-free radius of the electron source. δrS and δrC are the contributions of a spherical aberration (δrS) and a chromatic aberration (δrC). This leads to the following equation:
                    β        =                                            I              Beam                                                      π                2                            ·                              α                2                            ·                              (                                                                            r                      0                                        2                                    +                                      δ                    ⁢                                                                                  ⁢                                                                  r                        S                                            2                                                        +                                      δ                    ⁢                                                                                  ⁢                                                                  r                        C                                            2                                                                      )                                              .                                    [                  Equation          ⁢                                          ⁢          3                ]            
Accordingly, it would be desirable to provide an electron gun having a relatively good brightness and which may be operated under vacuum conditions which can be easily achieved (i.e., for example, at a residual pressure of about 10−6 or 10−7 mbar). Moreover, it would also be desirable to provide an electron beam device with such an electron gun as well as a method for controlling such an electron gun.